Electric discharge tube



Patented June 9, p 1931 UNITED STATES ALBERT CHARLES BARTLETT AND JoHniwALT n RYDE, for

ELECTRIC mana es} e s No Drawing. Application filed November 17, 1

This invention relates to electric gas discharge tubes and more particularly to cathodes for such tubes. i

' For many purposes it is important that @the potential required. to start a discharge through the tube and the potential required to maintain through it a discharge once started should both be as low as possible. 7 A low starting potential is desirable in order m that the discharge may start on the usual supply circuits; 2. low running potential is desirable in order that the energy dissipated in the tube when a given current passes through it shall be as small as possible. It is known that both these potentials can be reduced concurrently by substituting highly electropositive substances for ordinary metals as cathodes; accordingly it has been proposed to coat a cathode composed of ordinary metals with a layer of electropositive substances, such as the alkali or alkaline earth metals, or the alkaline earth oxides such as are used for thermionic emission. All such substances when exposed to the atmosphere 5 react with it to form less electropositive substances which havelittle or no effect in reducing the discharge potential. Accordingly in the manufacture of discharge tubes with such sensitive cathodes, it is necessary to prevent the access of air to the vesselbetween the formation of the sensitive surface and the final sealing off.

We have found that cathodes characterized by low running potentials can be prepared which will retain their properties, even after exposure to the atmosphere. Such cathodes can be made by causing suitable compounds of the alkaline earth metals to react with nickel, iron or certain of their alloys in virtue of a suitable heattreatment. But cathodes of this type are not sensitive in quite the same sense as those coated with the alkaline metals, because the starting potential is not appreciably lower than that obtained with starting potential is unimportant and only the running potential important. Thusin biphase rectifiers of the known type 1n Wl'llCll two anodes'are closely associated with the ordinary cathodes; only the running potential islower. However, for some purposes the Serial No.'234;030,andfin Great Britain November 1926. v

samecathode enclosed in a single vessel, the potential available to start the discharge is double thepeak potential of therectified sup ply,-and is often sufli'cient 'even when unsensitized 'cathodes are used. On the other hand/a low running. potential is important, because it implies a low energy loss in rectification. o According to the invention cathodes 'char- 'acterized by running potentials appreciably 1! zlowerthan those of the ordinary. metals and i not. appreciably higher than thoseofvthe 'alkali'metals and also stable whenexposedito the atmosphere are; "obtained "by" coating 1-1netallic cathodes ofnickel, iron or their alloys by. suitable" compounds ofthe' alkaline earthmetals. and subjecting them :to -a controlled heat .treatment, whereby the Comwpound reacts 1with.the. metallic cathodes forming a: new compound which contains :both the alkaline earth metaland thelnickel orlir on. I

It must be understood that the process is entirely difierent from that by which cathodes of tlie alkaline earth oxides are prepared, as (in. the original Wehnelt process, by heating an alkaline earthcompoundon a'metallicsupport.-v In the Wehnelt process the layerfremaining on the metallic: support was sub 'stantially pure alkalinegarth oxide and .did I not consist of i. a compound containing'fboth the alkaline earth metal and that .of the] support. The formation" ofusuch -complexl com- 1'; pounds isessentialtof-our; invention;

:Againzit' has; been suggested several 1 times thatrthe. success of various methodsiof pre-" aparing'lox-ide, cathodes for thermionic. dis

a charge vessels depends on a reaction between the oxide coating and the underlyingmetal.

' But it does not appear to have been suggested that cathodes prepared inthismanner would be valuable'in gas'fdischargle ives'sels, nor

would many of these methods be applicable a to bodies of the shape required.

according to this invention is as follows. The electrodes which may be made of nickel, are

A preferred methodof preparing cathodes g coated with a layer ofomoistened barium ni- I trate and'carefully dried by warming them.

The electrodes so coated are then placed .in a

vacuum furnace of any suitable form and heated carefully up to about 750 C. for about half an hour and then cooled. During the heatin process thebarium nitate coating re 5 acts with the nickel and changes its colour. The heating is so controlled that the coating produced is of a reddish brown colour. If the temperature to which the electrodes are, heated is too low the colour of the coating is usually black'and crystaline, whilst if the temperature is raised too high the colour is either lead grey or white and the coatings so producedare not satisfactory. The exact composition of the reddish brown barium compound has not been ascertained. Electrodes which have been produced by the con- 7 trolledheat treatment described above may be exposed to air during their assembly in electric discharge tubes without any'adverse '20 effect on atheir subsequent behaviour. To complete the manufacture of a discharge tube, the tube after assembly of the electrodes is evacuated and baked in a known manner, a charge of rare gas admitted and an electric discharge passed to heat the electrodes to a dull red heat until no further evolution of gas :occurs. The charge of gas is then pumped out, the tube filled with a suitable gas and sealed-off in the usual known manner. 80 Although it is preferable to carryout the controlled heat-treatment of the coated electrodes in a vacuum furnace, this is not essen- --tial, as it has been found that it is possible to obtain satisfactory results by heating the 85 icoatedele'ctrodes in'airr Further, although a nickel electrodes are preferred electrodes of nickel alloys, iron, or iron alloys may also be used. i V

We claim V 40 1. A process for the manufaotureof oath,-

odes for electric gas discharge tubes which comprises coating an electrode containing -nickelwith barium nitrate, subjecting the coated electrode to a controlled heat treat,- ment to partially decompose the said barium v .nitrate and to produce a compound which is neither gra-ynor black butreddish brown. I 2. The, manufacture of cathodes accorde 7 ing to claim 1, in which the heat treatment j v consists of heating for halfan hour to a tem- 'perature of 750 C. in a vacuum furnace. Y I 3. Cathodes for electric gas discharge tubes. comprising'a nickel cathode member 1 .fcoated with a reddish brown compound of barium, nickel and oxygen;

ALBERT CHARLES BARTLETT.

JOHN WALTER RYDE. 

